Investigation of the cBN/h-BN phase transformation at normal pressure

The phase transition of c-BN into h-BN was studied up to 1540 degrees C showing that c-BN is the stable modification at standard conditions. The transformation was examined by DTA (differential thermoanalysis) measurements. The morphology changes during conversion of c-BN into h-BN were investigated by SEM (scanning electron microscopy) indicating that the formation of h-BN preferably starts at the surface of the c-BN single crystals. The transformation step was furthermore characterized by IR and Raman spectroscopy as well as X-ray diffraction. The results have shown that a solid state and a gas phase mechanism have to be considered responsible for the phase conversion. The onset temperature of the phase transition from c-BN to h-BN depends significantly on grain size and impurities present in the samples. The lowest temperature for the beginning of the phase transformation was observed with a micron-size c-BN at 900 degrees C. (C) 1997 Elsevier Science S.A.